This invention relates generally to journal bearings for supporting horizontally oriented steam turbine shafts, and particularly to a three-pad journal bearing wherein the pads are of different arcuate lengths, are asymmetrical with respect to each other and with respect to the horizontal and vertical centerlines of the shaft, and each pad has associated therewith an oil feed dam immediately preceeding its leading edge.
In a large steam turbine, several sets of buckets are mounted on the steam turbine shaft and axially spaced-apart along the shaft to form a complete rotor. Each set of buckets, or each turbine stage, changes the enthalpy of the steam passing axially through the turbine which causes the rotor to rotate. The force of the steam admitted into the turbine affects the rotor. As is well known in the art, the direction and magnitude of this force is influenced by the particular control mode of operation for the turbine, i.e., full arc steam admission mode or partial arc mode. Hence, although the rotor primarily rotates about its axis, the turbine shaft also experiences both horizontal and vertical movements due to these forces.
Commonly a plurality of bearings are located at various axial locations along the shaft. Some bearings of a steam turbine include several pads which space the rotatable shaft away from the bearing casing. These bearings are normally lubricated with oil and some of this oil is distributed between each pad face and the shaft's surface. In operation, the oil in the interstice between the pad face and the shaft surface hydrodynamically lifts the shaft off the face of the pad. The amount of lift developed in the bearing determines the stiffness of the bearing to horizontal and vertical forces acting upon the shaft. In this manner, the bearing dampens the horizontal and/or vertical movements of the shaft, as well as, rotatably supports the shaft without placing large frictional forces thereon which inhibit the rotation of the shaft. The frictional forces inherent within the bearing, and hence power losses, are minimized by the oil film in the interstice defined by the rotating shaft surface and the face of the pad. Additionally, the oil film cools the pad face, which is heated by friction, thereby protecting the integrity of the bearing.
Due to the great weight of the turbine carried by the shaft in combination with the speed of rotation of the shaft, a bearing which loses this oil film in one or all of its interstices deterioriates rapidly because the shaft surface wipes the pad face and, consequently, the shaft and/or the pad face may be scored. The resulting inefficiency of a wiped bearing is well known in the art. Additionally, when the shaft surface does come in wiping contact with the pad face, great frictional forces are generated by that contact which affects the immediate performance of the steam turbine.
Since horizontal and vertical damping of the turbine shaft motion is an important function of the bearing, three pad bearings have been developed. The three pad bearing lessens the total amount of pad face area which interacts with the shaft surface, thereby lowering the total viscous shear of the oil, and hence, lowering the total frictional forces and power losses developed within the bearing. However, the minimization of the shaft surface/pad face interface introduces arcuate spaces between each pad, i.e., the space defined by the trailing edge of the preceeding pad, the leading edge of the next or succeeding pad, the shaft surface and the radially inner surface of the bearing casing.
Since the lubricating oil cools the pad face, as well as provides support for the shaft, a continuous stream of oil normally flows through the interstice between each pad face and the adjacent shaft surface. The oil is ejected proximate the trailing edge of each pad. The ejected oil churns within the arcuate space and such churning is believed to cause some power loss in the bearing.
Although means for draining or evacuating the "hot" oil, i.e., the oil ejected from the preceeding pad's interstice, from these arcuate spaces could be improved, a problem still exists in supplying the next or succeeding pad's interstice with enough cool, lubricating oil to maintain the oil film which continuously flows in the latter interstice. This particular problem is termed "hot oil carry-over" herein. One prior art device, described in U.S. Pat. No. 4,247,157, issued to Sigg, sprays oil preceding nozzles, located along the axial length of each pad, towards the leading edge of the pad face. The prior art device also includes a shield affixed to the nozzle which has a radially inner edge, proximate the rotating shaft surface, which shields the fresh lubricating oil spray from the oil ejected from the preceeding pad. However, the prior art device does not address the problem of oil churning in the arcuate spaces and mixture of the fresh lubricating oil with the hot oil carried-over from the preceeding pad. Additionally, this prior art device may not be operable with the low pressure supply of lubricating oil commonly utilized in large steam turbine lubricating systems (commonly 10 to 15 pounds per square inch) because the oil in the patented device must, by necessity, be under relatively high pressure to be sprayed from the nozzle. Aeration of the oil may also present a problem.